Magnetic field generators are used to generate alternating magnetic fields for domestic or industrial induction heating uses or hyperthermia treatments. An alternating magnetic field is produced by passing an alternating current through a coil. Generators of large alternating magnetic fields employ the resonant principle, in which energy passes back and forth at the characteristic resonant frequency between the magnetic field associated with current in a coil and the electric field associated with voltage across a capacitor. The advantage of this technique is that the power source need only compensate for losses, rather than supplying the full magnetic field energy on every cycle. There are two standard circuit configurations used in this technique: series resonant and parallel resonant.
In a series resonant system, the coil is made to operate at the intended frequency by a capacitor placed in series with it. In this configuration, the entire current through the coil also flows in the power source, which becomes a limiting factor. To obtain a large field (in volume and/or intensity) using a moderate current flow requires a coil of many turns, which has high inductance. The effect of the series resonance is then to create an extremely high voltage across the coil, possibly reaching tens of kilovolts, which is a problem for insulation and capacitor rating, and can even result in corona discharge in the air. The series resonant configuration is readily driven by a simple voltage source. However, because of engineering and safety considerations related to high voltage and the impracticality of operation at high frequencies (for example, above 100 kHz), series resonance is not preferred for generating large high-frequency magnetic fields.
In a parallel resonant system, the coil is made to operate at the intended frequency by a capacitor placed in parallel with it. In this configuration, the resonance amplifies the current flowing in the coil to far higher levels than the capability of the power source. A large field can be created with a coil of few turns and relatively low inductance such that the voltage is much lower than for series resonance, and higher frequency operation becomes practicable. This is the preferred configuration for generating large, high-frequency magnetic fields, but has the drawback that conventionally inductive components (matching inductors or high-frequency transformers) are used in providing an effective current source to drive it. These components add to complexity and power loss, and may also result in damaging voltage spikes if the power source becomes de-tuned from the resonance.
What is required is an efficient, scalable driver that is able to produce a high magnetic field at high frequency in a low inductance coil.
The present invention addresses this need.